The present invention relates to a control system for an internal combustion engine having a compression ratio control mechanism capable of varying a compression ratio, a lift and operation angle control mechanism capable of varying a lift and operation angle of an intake valve, and a phase control mechanism capable of varying a maximum lift phase of an intake valve. The present invention further relates to a method for controlling such an internal combustion engine. Still further, the present invention relates to a technique for accelerating warm-up of a spark-ignited gasoline engine.
Heretofore, the assignee of this application proposed various compression ratio control mechanisms for reciprocating internal combustion engines, utilizing a double-link type piston-crank mechanism and adapted to vary the top dead center (TDC) of the piston by moving a portion of a linked structure thereof as disclosed in Japanese Patent Provisional Publication No. 2002-21592. This kind of compression ratio control mechanism varies a mechanical compression ratio of an internal combustion engine, i.e., a nominal compression ratio and generally controls the compression ratio so that a high compression ratio is obtained at partial load for improving the thermal efficiency and a low compression ratio is obtained at high load for avoiding engine knock.
Further, the assignee of this application has proposed a variable valve timing control mechanism that can vary the lift and operation angle of an intake valve simultaneously and continuously and a variable valve timing control mechanism that can attain a wide design freedom of the lift characteristics in combination with a phase control mechanism for varying a maximum lift phase as disclosed in Japanese Patent Provisional Publication Nos. 2002-89303 and 2002-89341.
As is well known, an exhaust system of an internal combustion engine is provided with a catalytic exhaust gas purifier including an oxidation-reduction catalyst, oxidation catalyst or a reduction catalyst. However, even by the advanced catalyst technology of today, it is the present situation that the catalyst effect on the gas emitted from the engine is largely limited immediately after cold start at which the catalyst temperature is low as compared with that after warm-up of the engine. This problem has long been recognized by the person skilled in the art. Thus, a continuous effort has been made to lower the active temperature of the catalyst and it has been devised to introduce the secondary air into the upstream side of the catalyst for thereby accelerating the time at which the catalyst is chemically activated.
However, the problem basically depends upon how fast the catalyst can reach the temperature at which the catalyst starts conversion. To this end, the ignition timing is delayed during warm-up (i.e., the exhaust gas temperature is elevated by retarding the timing at which combustion starts). This causes a bad influence on the fuel consumption but is widely exercised. However, to elevate the exhaust gas temperature only by retarding the ignition timing has its limit.
On the other hand, paying attention to an expansion ratio of an internal combustion engine (i.e., the ratio of cylinder volume at the time the exhaust valve is open to the cylinder volume at TDC), a higher expansion ratio is desirable for maximizing the effective work of the combustion gas and a lower expansion ratio is desirable for making higher the exhaust gas temperature for accelerating activation of the catalyst. By lowering the expansion ratio and thereby lowering the ratio at which the energy of the combustion gas is converted to the work, the exhaust gas temperature can be elevated. In this connection, advance of the opening timing of the exhaust valve can produce an exhaust gas temperature elevating effect since the combustion gas in the middle of expansion can be discharged in an early stage. However, since the combustion gas in a state of being high in pressure is discharged through the exhaust valve, a considerable amount of heat is taken away from the combustion gas due to transmission of heat by a portion of the engine around the exhaust valve and therefore the exhaust gas temperature cannot be elevated efficiently.
It is accordingly an object of the present invention to provide a control system for an internal combustion engine having a compression ratio control mechanism, a lift and operation angle control mechanism and a phase control mechanism, which can raise the exhaust temperature efficiently by lowering the geometrical expansion ratio of the engine by means of the compression ratio control mechanism while at the same time by retarding the ignition timing.
It is a further object of the present invention to provide a method for controlling an internal combustion engine, which is carried out by the control system of the foregoing character.
According to an aspect of the present invention, there is provided a control system for an internal combustion engine having a compression ratio control mechanism capable of varying a compression ratio of the engine by varying a top dead center position of a piston and an ignition timing control system capable of varying an ignition timing of the engine, the control system comprising an engine control unit for controlling the compression ratio control mechanism and the ignition timing control system so that the compression ratio is varied depending upon variations of engine rpm, engine load and a warm-up condition of the engine and that when the engine is cold, the ignition timing is retarded from a MBT point and the piston top dead center position is made lower in level than that obtained when the engine is hot and operated at corresponding engine rpm and engine load.
According to another aspect of the present invention, there is provided an internal combustion engine comprising a compression ratio control mechanism capable of varying a compression ratio of the engine by varying a top dead center position of a piston, an ignition timing control system capable of varying an ignition timing of the engine, and a control unit for controlling the compression ratio control mechanism and the ignition timing control system so that the compression ratio is varied depending upon variations of engine rpm, engine load and a warm-up condition of the engine and that when the engine is cold, the ignition timing is retarded from a MBT point and the piston top dead center position is made lower in level than that obtained when the engine is hot and operated at corresponding engine rpm and engine load.
According to a further aspect of the present invention, there is provided a method for controlling an internal combustion engine having a compression ratio control mechanism capable of varying a compression ratio of the engine by varying a top dead center position of a piston and an ignition timing control system capable of varying an ignition timing of the engine, the method comprising controlling the compression ratio control mechanism and the ignition timing control system so that the compression ratio is varied depending upon variations of engine rpm, engine load and a warm-up condition of the engine and that when the engine is cold, the ignition timing is retarded from a MBT point and the piston top dead center position is made lower in level than that obtained when the engine is hot and operated at corresponding engine rpm and engine load.